The present invention relates generally to ceramic and metal composites and more specifically to silicon nitride- metal composites.
The U.S. Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the U.S. Department of Energy and the University of California, for the operation of Lawrence Livermore National Laboratory.
It has long been recognized that a combination of the heat and corrosion resistant properties of ceramics, with the heat condition and durability of metals, would be highly desirable for many applications such as for instance, in internal combustion engines and in other energy conversion systems. In recent years, therefore, cermets, which are composites of ceramics and metals, have been widely researched for use as high temperature, corrosion-resistant and high strength component materials in rocket nozzles, gas turbines, high temperature furnaces, helical expanders and the like. Silicon nitride-based ceramic materials are being considered for use in Sterling engines and high temperature diesel engines. A strong, thermally stable bond between the two dissimilar materials, ceramics and metals, is, however, difficult to achieve because of the brittleness and the low coefficient for thermal expansion of the ceramics. Various techniques for bonding these ceramic materials with metals or other ceramics have been widely and extensively investigated, to increase the utility of these ceramic-metal composites for high temperature uses.
U.S. Pat. No. 4,347,089 issued to Loehman discloses a method for bonding silicon nitride materials using silicon oxynitride.
U.S. Pat. No. 4,469,658 issued to Kimura et al., describe sintered silicon carbide composites containing Fe, Ni or Co.
U.S. Pat. No. 4,505,418 issued to Neidig et al., discloses a method for directly bonding copper foils to ceramic substrates.
U.S. Pat. No. 4,055,451 issued to Cockbain et al., teaches composite ceramic-metal or ceramic-ceramic, and a method for fabricating the same.
U.S. Pat. No. 4,492,737 issued to Connolly, discloses a composite consisting of a ceramic and a metallic component.
U.S. Pat. No. 4,293,619 issued to Landingham et al., teaches a composite which includes silicon nitride, a refractory metal and a layer of MoSi.sub.2, and a method for making the same. The method essentially consists of contacting as a bonding agent, a layer of MoSi.sub.2 with a portion of the silicon nitride substrate and with a portion of the refractory metal substrate, heating the MoSi.sub.2 layer to a temperature of about 1000.degree. C. to about 1300.degree. C., and simultaneously with the heat, compressing the silicon nitride and refractory metal layers at pressures in excess of about 30 MPa. The metals used are molybdenum, titanium and zirconium.
U.S. Pat. No. 4,131,459 issued to Fletcher et al., discloses cermet compositions suitable as high temperature resistant refractory coatings on molybdenum and tungsten substrates.
U.S. Pat. No. 4,325,647 issued to Maier et al., describes ceramic-metal composite with a connecting element consisting of an insulating, resilient body of a ceramic material.
U.S. Pat. No. 4,470,537 issued to Diem et al., teaches a solid state method for bonding ceramic and metal parts.
A sintered composite of boron nitride and a metal and a method of making the composite is taught in U.S. Pat. No. 4,394,170 issued to Sawaoka et al.
U.S. Pat. No. 4,404,262 issued to Watmough, describes a method for making a composite of a ceramic material and a metallic component.
Thus, in spite of a great deal of activity in this area, current techniques for bonding metals to ceramic materials have not been very successful in producing strong, heat and corrosion resistant, machinable composites, particularly involving hard metals like iron and steel. The composites produced by these prior art methods are usually brittle and nondeformable and therefore, break down under thermal cycling. These difficulties stem mainly from a mismatch in their thermal expansion properties and from the great differences in other physical and chemical properties such as in the coefficients for thermal conductivity and expansion between the dissimilar metals and the ceramic materials.
Accordingly, it is an object of the present invention to provide a ceramic-metal composite whwich exhibits low expansion and high temperature bonding.
Another object of the invention is to provide a multilayered silicon nitride-stainless steel composite.
Yet another object is to provide a bond of a metal shaft, more specifically a stainless steel shaft, to a ceramic turbine rotor.
Still another object is to provide a two stage technique for creating a strong ceramic-metal bond.
Additional objects, advantages and novel features of the invention, together with additional features contributing thereto and advantages accruing therefrom will be apparent from the following description and the accompanying illustration of an embodiment of the invention and the description of the fabrication technique therefor, as described hereinafter. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.